1. Field of the Invention
This invention relates generally to methods for fracturing subterranean formations penetrated by a well bore utilizing a crosslinked aqueous gel which is injected into the formation through a suitable conduit at a rate and pressure sufficient to fracture the formation.
2. Description of the Prior Art
In fracturing oil and gas wells, a special fluid referred to as a fracturing fluid is pumped down the well to contact the formation to be fractured. The fluid can contain proppant materials such as sand, glass beads, nylon pellets, or the like. The pressure of the fluid composition is increased by pumps located at the surface until the formation is fractured by hydraulic pressure. Fracturing fluid is pumped at a rate sufficient to open a fracture in the exposed formation and extend the fracture from the well bore into the formation. Continued pumping of the fracturing fluid containing a propping agent into the fracture results in proppant placement within the fractured zone. Following the treatment, the fracturing fluid is recovered from the well, leaving the proppant remaining in the fracture, thereby preventing the complete closure of the formation and forming a permeable channel extending from the well bore into the formation.
One fracturing fluid which is in use in fracturing operations includes a guar gum and water. When these two materials are mixed, a viscous gel is formed. The guar gum is usually added to the water in the form of a dry powder which hydrates with time to form a viscous gel. The actual viscosity which is achieved is dependent upon the relative proportions of water and gum used.
While adding increased amounts of gum to the base fluid in one technique for increasing the viscosity of the fluid, such practice quickly becomes uneconomical because of the cost of the gum. A more efficient practice involves the use of crosslinking agents. These agents can be used in very small amounts to give large increases in the apparent viscosity of the base fluid. Some common crosslinking agents include borates, aluminum, and the so called organotitanates.
In the past, it was thought that the gum must be permitted to hydrate for an appreciable length of time in the aqueous fluid prior to the addition of the crosslinking agent. As a result, one typical prior art technique involved the "batch mixing" of the gum and water by supplying water from a frac tank to a blender and adding the gum to the blender. A portion of the product from the agitated blender was drawn off and recirculated to the frac tank until the polymer was fully hydrated. At this point, the aqueous fluid containing the hydrated polymer was drawn off and passed to a blender where proppant was added to the fluid. The fracturing fluid was then flowed toward the wellhead and crosslinker was added to the fluid conduit so that the hydrated polymer was crosslinked on its way to the formation to be fractured. Although this was a workable technique, it is often desirable to provide a continuous fracturing method which can be utilized to introduce an aqueous fluid into the borehole, and in which gelling agent can be hydrated and crosslinked without an appreciable "residence time" in a frac tank or other holding vessel.
The "continuous" prior art techniques which are known to applicants have typically utilized a blender to combine the aqueous fluid and gum. The base gel formed in the blender is passed to an intermediate holding tank which provides a certain "residence time" to allow complete hydration of the polymer. Currently, about a two minute residence time is the minimum required. Lesser residence time is not recommended because the addition of conventional crosslinkers will inhibit the hydration rate of the polymer thus producing an inferior fracturing gel. In other words, when the residence time is too short, full hydration of the polymer is not obtained prior to crosslinking. If conventional crosslinkers are mixed with the partially hydrated polymer, further hydration is not obtained and the viscosity of the resulting fracturing fluid is lower than the optimum value.
The residence time of the polymer in the holding vessel depends upon the capacity of the vessel and the injection rate in a continuous fracturing process. For example, if the capacity of the holding vessel is 25 barrels and the injection rate is 25 barrels per minute, the residence time is one minute. In order to obtain a two minute residence time, it would be necessary to have a holding vessel with a capacity of 50 barrels or a lower injection rate. Although the capacity of the holding vessel is not critical on land, it is desirable to reduce this vessel size on offshore locations.
It is an object of the present invention to provide a continuous mix process for preparing an aqueous fracturing gel in which the size of the holding vessel can be reduced to a minimum, or in which the holding vessel can be eliminated entirely.
Another object of the invention is to provide a continuous fracturing method utilizing a gum and a crosslinker which allows the gum to continue to hydrate in the presence of the crosslinker.